Solenoid Pump Driver

ABSTRACT

A method of driving a solenoid pump of the type comprising a metal shuttle urged by a solenoid against a spring with the spring providing the force for the pumping stroke of the shuttle is disclosed. The method comprises applying a periodic driving voltage to the solenoid. Each period of the driving voltage comprises a first portion during which the driving voltage increases from a minimum voltage to a maximum voltage to compress the spring, a second portion during which the driving voltage decreases from the maximum voltage to the minimum voltage to release the spring, and a third portion during which the driving voltage is maintained at the minimum voltage. The duration of the second portion is substantially less than the duration of the first portion and may be substantially instantaneous. The driving voltage increases from the minimum voltage to the maximum voltage substantially linearly during the first portion, such that the driving voltage has a sawtooth waveform.

This invention relates to a method and apparatus for driving anelectromagnetic pump.

BACKGROUND

FIG. 1 is a sectional view of a known construction of an electromagnetic(or solenoid) pump, for example as shown in U.S. Pat. No. 9,028,227. Thepump is configured to pump fluid, such as water, from an inputconnection 1 to an output connection 2 in the direction of the arrow A.The input connection 1 is formed as a plastics moulding, which defines achamber in which is received a power spring 3 and a shuttle 4. Anelectrical solenoid 5 surrounds the chamber. An outer magnetic core 6surrounds the solenoid 5. A non-magnetic spacer 7 breaks the magneticcircuit of the outer magnetic core 6. The shuttle 4 is made of stainlesssteel and is therefore moveable magnetically by the solenoid 5. Theshuttle 4 provides a core and a piston. The core is received within thechamber and engages the power spring 3 at its proximal end. The pistonextends from the core and is received within a cylinder 8. The shuttle 4is hollow and provides a fluid passage through the core and the piston.The fluid passage is closed at the distal end of the piston by a pistonnon-return valve 9, which is retained in position by a spring within thefluid passage. A retaining washer 10 locates the piston within thecylinder 8 and a sealing O-ring 11 seals the proximal end of thecylinder 8. A non-return valve 12 is provided between the cylinder 8 andthe output connection 2 to seal the distal end of the cylinder 8. Asecondary spring 13 is provided between the core and the distal wall ofthe chamber to cushion movement of the shuttle 4.

In operation, the solenoid 5 is energised electrically, which moves thecore (and thus the entire shuttle 4) towards the input connection 1,compressing the power spring 3. The shuttle 4 is moved up to and betweenthe gap created in the outer magnetic core 6 by the non-magnetic spacer7. The movement of the shuttle 4 reduces the pressure within thecylinder 8, which is closed by the non-return valve 12. The reducedpressure in the cylinder 8 opens the piston non-return valve 9, whichallows fluid to pass from the input connection 1 through the chamber andthe fluid passage within the shuttle 4 into the cylinder 8. On thereturn stroke of the pump, the power spring 3 pushes the core (and thusthe entire shuttle 4) towards the output connection 1. The pistonnon-return valve 9 closes against the pressure of the fluid in thecylinder 8 and this pressure causes the non-return valve 12 to open sothat the piston forces the fluid out of the cylinder 8 through theoutput connection 2 under the action of the power spring 3.

Although the presently described pump uses the electromagnet to fill thecylinder 8 and load the power spring 3, and uses the loaded power spring3 to force the fluid out of the cylinder 8 through the output connection2, it is known to have a pump operating in the opposite sense. That is,it is known to use the spring to draw fluid into the cylinder, and usethe electromagnet to force the fluid out of the cylinder through theoutput connection.

Typically, the electrical solenoid 5 is driven by a half-wave rectifiedvoltage at mains frequency (50 Hz in Europe), which provides a simpledrive voltage signal to the solenoid 5.

The present invention, at least in its preferred embodiments, providesan alternative method of driving a solenoid pump.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present invention there is provided a method ofdriving a solenoid pump of the type comprising a metal shuttle urged bya solenoid against a spring with the spring providing the force for apumping stroke of the shuttle, for example the solenoid pump describedwith reference to FIG. 1. The method comprises applying a periodicdriving voltage to the solenoid. Each period of the driving voltagecomprises a first portion during which the driving voltage increasesfrom a minimum voltage to a maximum voltage to compress the spring, asecond portion during which the driving voltage decreases from themaximum voltage to the minimum voltage to release the spring, and athird portion during which the driving voltage is maintained at theminimum voltage. The duration of the second portion is substantiallyless than the duration of the first portion.

Thus, in accordance with the invention, the driving voltage is reducedrelatively quickly from the maximum voltage to the minimum voltageduring the second portion. In this way, the spring is quickly freed toprovide the force for the pumping stroke of the shuttle with the minimumenergy being wasted by the driving voltage causing the solenoid to actagainst the released spring.

The driving voltage may decrease from the maximum voltage to the minimumvoltage gradually, for example linearly. The duration of the secondportion may be less than half of the duration of the first portion,particularly less than 10% of the duration of the second portion. In aparticular embodiment, the driving voltage may decrease from the maximumvoltage to the minimum voltage substantially instantaneously during thesecond portion. Thus, the duration of the second portion may benegligible compared to the duration of the first portion.

In embodiments of the invention, the driving voltage increases from theminimum voltage to the maximum voltage substantially linearly during thefirst portion. Although this configuration is presently preferred, it ispossible for the driving voltage to increase otherwise than linearlyduring the first portion. In embodiments of the invention, the drivingvoltage has a sawtooth waveform.

The method may comprise controlling the duration of the third portion toprovide a required flow rate through the pump. Alternatively or inaddition, the method may comprise controlling the maximum voltage toprovide a required flow rate through the pump.

The frequency of the driving voltage may be different to the frequencyof a supply voltage providing electrical power for the driving voltage.Thus, the frequency of the supply voltage is not limited to 50 Hz/60 Hz.The method may therefore comprise controlling the frequency of thedriving voltage to provide a required flow rate through the pump.

Typically, the minimum voltage is zero volts.

The invention extends to a driver circuit for a solenoid pump, thedriver circuit configured to generate a driving voltage in accordancewith the method of the invention. The driver circuit may be an analoguecircuit, a digital circuit or a combination of analogue and digitalcircuitry. The driver circuit may use pulse width modulation (PWM) togenerate a sawtooth, or other, waveform. The invention further extendsto a solenoid pump in combination with the driver circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 is a sectional view of an electromagnetic pump;

FIG. 2 is a graph showing a comparison between a known driving methodfor the electromagnetic pump of FIG. 1 and a driving method according toan embodiment of the present invention;

FIG. 3 is a graph illustrating a driving method according to a furtherembodiment of the present invention;

FIG. 4 is a graph illustrating a driving method according to a yetfurther embodiment of the present invention; and

FIG. 5 is a schematic of a driving circuit according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

FIG. 2 shows, in the upper graph, a comparison of a sawtooth drivingvoltage for a solenoid pump according to an embodiment of the invention(dashed line) to a half-wave rectified driving voltage of the prior art(solid line). The solenoid pump may be of the type described in relationto FIG. 1. The lower graph in FIG. 2 shows the corresponding springforce of the power spring 3. As shown in FIG. 2, to achieve the samespring force from the power spring 3, the sawtooth driving voltagerequires only 60% of the maximum voltage of the rectified drivingvoltage. Furthermore, the rapid decrease of the sawtooth driving voltagefrom the (60%) maximum voltage to zero volts, also saves energy. Theamount of energy saved by using the sawtooth driving voltage is shadedin FIG. 2.

FIG. 3 shows a variation of the sawtooth driving voltage of anembodiment of the invention in a representation corresponding to FIG. 2.In this case, the first portion of the sawtooth waveform has a durationof 10 ms, as in the waveform of FIG. 2. The second portion of thesawtooth waveform, in which the driving voltage drops from a maximumvalue to zero volts is substantially instantaneous. In FIG. 3, theduration of the third portion of the sawtooth waveform in which thedriving voltage is maintained at zero volts is reduced relative to thewaveform shown in FIG. 2 from 10 ms to 8 ms. In this way, the frequencyof the driving voltage is increased from 50 Hz to 56 Hz. The shorterduration of each period of the sawtooth waveform increases the flow ratethrough the pump.

FIG. 4 shows a variation of the sawtooth driving voltage of anembodiment of the invention in a representation corresponding to FIGS. 2and 3. In this case, the first portion of the sawtooth waveform has aduration of 8 ms, which is shorter than in the waveform of FIGS. 2 and3. The rate of change (gradient) of the driving voltage remains thesame, however, such that the maximum voltage achieved during the firstportion of the waveform is reduced, compared to FIGS. 2 and 3. Again,the second portion of the sawtooth waveform, in which the drivingvoltage drops from a maximum value to zero volts is substantiallyinstantaneous. In FIG. 4, the duration of the third portion of thesawtooth waveform in which the driving voltage is maintained at zerovolts is longer than in the waveforms of FIGS. 2 and 3 at 12 ms. In thisway, the frequency of the driving voltage is maintained at 50 Hz, as inFIG. 3, but the flow rate through the pump is reduced as the powerspring 3 only reaches 80% of its maximum compression.

The sawtooth driving voltage has the advantage that it uses less energyto achieve the same spring force than a conventional driving voltage.This means that a smaller power supply can be used and less heat isgenerated during operation of the pump, which increases the pump dutycycle. Furthermore, the operating frequency of the pump can be “tuned”to achieve the minimum mechanical noise from the pump components. Forexample, we have found that known pumps can operate at their quietest ata driving frequency of 47 Hz, rather than the conventional 50 Hz. Anysuitable driver circuit may be used to generate the required sawtoothwaveform.

FIG. 5 is a schematic of a driving circuit according to an embodiment ofthe present invention. The driving circuit 20 comprises a power supply22 connecter to a microcontroller 24. The microcontroller is connectedto a pump solenoid 28 via a switching circuit 26. The pump solenoid istypically the solenoid 5 as described in FIG. 1. The microcontroller 24comprises a memory and at least one processor. The memory of themicrocontroller 24 includes instructions which, when executed, cause theat least one processor to control the switching circuit 26 and the pumpsolenoid 28 to operate in accordance with methods as describedpreviously. The switching circuit 26 provides inputs for themicrocontroller 24 to control the operation of the pump solenoid 28.

In summary, a method of driving a solenoid pump of the type comprising ametal shuttle urged by a solenoid against a spring with the springproviding the force for the pumping stroke of the shuttle is disclosed.The method comprises applying a periodic driving voltage to thesolenoid. Each period of the driving voltage comprises a first portionduring which the driving voltage increases from a minimum voltage to amaximum voltage to compress the spring, a second portion during whichthe driving voltage decreases from the maximum voltage to the minimumvoltage to release the spring, and a third portion during which thedriving voltage is maintained at the minimum voltage. The duration ofthe second portion is substantially less than the duration of the firstportion and may be substantially instantaneous. The driving voltageincreases from the minimum voltage to the maximum voltage substantiallylinearly during the first portion, such that the driving voltage has asawtooth waveform.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to”, and they are not intended to (and do not) exclude othercomponents, integers or steps. Throughout the description and claims ofthis specification, the singular encompasses the plural unless thecontext otherwise requires. In particular, where the indefinite articleis used, the specification is to be understood as contemplatingplurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics or groups described in conjunctionwith a particular aspect, embodiment or example of the invention are tobe understood to be applicable to any other aspect, embodiment orexample described herein unless incompatible therewith. All of thefeatures disclosed in this specification (including any accompanyingclaims, abstract and drawings), and/or all of the steps of any method orprocess so disclosed, may be combined in any combination, exceptcombinations where at least some of such features and/or steps aremutually exclusive. The invention is not restricted to the details ofany foregoing embodiments. The invention extends to any novel one, orany novel combination, of the features disclosed in this specification(including any accompanying claims, abstract and drawings), or to anynovel one, or any novel combination, of the steps of any method orprocess so disclosed.

1. A method of driving a solenoid pump of the type comprising a metalshuttle urged by a solenoid against a spring with the spring providingthe force for a pumping stroke of the shuttle, the method comprisingapplying a periodic driving voltage to the solenoid, wherein each periodof the driving voltage comprises a first portion during which thedriving voltage increases from a minimum voltage to a maximum voltage tocompress the spring, a second portion during which the driving voltagedecreases from the maximum voltage to the minimum voltage to release thespring, and a third portion during which the driving voltage ismaintained at the minimum voltage, wherein the duration of the secondportion is substantially less than the duration of the first portion. 2.A method as claimed in claim 1, wherein the driving voltage decreasesfrom the maximum voltage to the minimum voltage substantiallyinstantaneously during the second portion.
 3. A method as claimed inclaim 1, wherein the driving voltage increases from the minimum voltageto the maximum voltage substantially linearly during the first portion.4. A method as claimed in claim 1, wherein the driving voltage has asawtooth waveform.
 5. A method as claimed in claim 1 further comprisingcontrolling the duration of the third portion to provide a required flowrate through the pump.
 6. A method as claimed in claim furthercomprising controlling the maximum voltage to provide a required flowrate through the pump.
 7. A method as claimed in claim 1, wherein thefrequency of the driving voltage is different to the frequency of asupply voltage providing electrical power for the driving voltage.
 8. Amethod as claimed in claim 1 further comprising controlling thefrequency of the driving voltage to provide a required flow rate throughthe pump.
 9. A method as claimed in claim 1, wherein the minimum voltageis zero volts.
 10. A driver circuit for a solenoid pump, the drivercircuit configured to generate a driving voltage in accordance with themethod of claim
 1. 11. A solenoid pump in combination with the drivercircuit of claim 11.